48
Spectroscopic Nutrient Measurements
An alternate avenue for nitrate analysis is afforded by the marked optical absorption
peak of this ion in solution, an attractive analytical alternative since reagents need
not be added. Commercial instruments based on this principle are available (Johnson
and Coletti 2002). Interference of naturally occurring, optically active, dissolved
inorganic bromide and bisulfide ions has been largely overcome by measurement of
seawater absorption in 35 optical channels within the narrow wavelength band of
217–240 nm followed by multivariate linear regression. Optical opacity of glass at
these wavelengths dictates the use of quartz optical windows. Further accuracy is
achieved by accounting for instrument dark intensity; readings observed in the
absence of irradiation (Martin et al. 2017). However, as noted below, absorption by
colored dissolved organic matter (CDOM) is minimal in the red portion of the spectrum but increases exponential through the blue and UV causing substantial interference to UV nitrate determinations in waters with high CDOM content such as
coastal and estuarine waters (Edwards et al. 2001; MacIntyre et al. 2009). Variability
of the CDOM absorption spectrum as a result of both the molecular nature of the
material and photodegradation suffered in transit has further hampered efforts to
develop algorithms capable of discriminating the contribution of CDOM to the total
absorption signature. Instrument accuracy is typically above 2 μM (Martin et al.
2017), making the method unusable for oligotrophic waters of low nutrient
content.
The Alliance for Coastal Technologies recently (2017) announced the results of a
Nutrient Sensor Challenge carried out under the auspices of a coalition of private,
academic, and government organizations. Of 29 registered participants 5 were selected
for the field phase of the challenge. It is encouraging to note that, while the winner was
a conventional lab-in-a-can flow injection instrument, an honorable mention was
extended to the manufacturers of a lab-on-a-chip microfluidic device. Through continued challenges, ACT seeks to promote the development of instruments providing
precise and accurate phosphate and nitrate measurements and reliably surpassing a
3 month deployment threshold.
2.4 Electro-Optical Sensors for Measurement of Organic
Matter in Seawater
2.4.1 Colored Dissolved Organic Matter in Seawater
Chemical Nature, Distribution, and Pertinence of CDOM to Coastal Ocean
Observing
Dissolved organic matter (DOM) in natural waters may or may not interact with
light in the UV/Vis spectrum. Analysis of DOM is largely performed in the laboratory using the amount of dissolved organic carbon (DOC) in seawater as an index of
2 Electronic Sensors and Instruments for Coastal Ocean Observing
Spectroscopic Nutrient Measurements
An alternate avenue for nitrate analysis is afforded by the marked optical absorption
peak of this ion in solution, an attractive analytical alternative since reagents need
not be added. Commercial instruments based on this principle are available (Johnson
and Coletti 2002). Interference of naturally occurring, optically active, dissolved
inorganic bromide and bisulfide ions has been largely overcome by measurement of
seawater absorption in 35 optical channels within the narrow wavelength band of
217–240 nm followed by multivariate linear regression. Optical opacity of glass at
these wavelengths dictates the use of quartz optical windows. Further accuracy is
achieved by accounting for instrument dark intensity; readings observed in the
absence of irradiation (Martin et al. 2017). However, as noted below, absorption by
colored dissolved organic matter (CDOM) is minimal in the red portion of the spectrum but increases exponential through the blue and UV causing substantial interference to UV nitrate determinations in waters with high CDOM content such as
coastal and estuarine waters (Edwards et al. 2001; MacIntyre et al. 2009). Variability
of the CDOM absorption spectrum as a result of both the molecular nature of the
material and photodegradation suffered in transit has further hampered efforts to
develop algorithms capable of discriminating the contribution of CDOM to the total
absorption signature. Instrument accuracy is typically above 2 μM (Martin et al.
2017), making the method unusable for oligotrophic waters of low nutrient
content.
The Alliance for Coastal Technologies recently (2017) announced the results of a
Nutrient Sensor Challenge carried out under the auspices of a coalition of private,
academic, and government organizations. Of 29 registered participants 5 were selected
for the field phase of the challenge. It is encouraging to note that, while the winner was
a conventional lab-in-a-can flow injection instrument, an honorable mention was
extended to the manufacturers of a lab-on-a-chip microfluidic device. Through continued challenges, ACT seeks to promote the development of instruments providing
precise and accurate phosphate and nitrate measurements and reliably surpassing a
3 month deployment threshold.
2.4 Electro-Optical Sensors for Measurement of Organic
Matter in Seawater
2.4.1 Colored Dissolved Organic Matter in Seawater
Chemical Nature, Distribution, and Pertinence of CDOM to Coastal Ocean
Observing
Dissolved organic matter (DOM) in natural waters may or may not interact with
light in the UV/Vis spectrum. Analysis of DOM is largely performed in the laboratory using the amount of dissolved organic carbon (DOC) in seawater as an index of
2 Electronic Sensors and Instruments for Coastal Ocean Observing